Employee Benefits - Postretirement Benefits Other Than Pension Expenses (Detail) - Postretirement Benefit Plan [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 301	$ 323	$ 601	$ 645
Interest cost on projected benefit obligation	1,082	1,238	2,164	2,475
Net amortization and deferrals	(712)	(979)	(1,423)	(1,958)
Total postretirement benefit expense	$ 671	$ 582	$ 1,342	$ 1,162